We continue to study the biology as well as the clinical value of the BB isoenzyme of creatine kinase (CK-BB) using a radioimmunoassay developed in our laboratory. We have found that CK-BB is a marker for small cell carcinoma of the lung and that serum levels seem to reflect the amount of tumor burden present. On the other hand, CK-BB does not appear to be useful as a marker for prostatic carcinoma based on our prospective longitudinal study of patients with the disease. New studies we have done of crossreactivity indicate that our assay measures both the BB and MB isoenzyme of CK, and that both are elevated in serum following myocardial infarction.